Long-held teachings that hyaluronate-dependent viscosity accounts for the lubricating ability of synovial fluid are strongly contradicted by several sorts of recent evidence. We propose to use an in vitro test system for studying two aspects of "boundary" lubrication by synovial fluid: 1. The physicochemical nature of surfaces which can be lubricated by synovial fluid; 2. The lubricating ability of synovial fluid in various physiological and disease states. Lubrication will be compared with the dynamic viscosity of the fluid using a miniaturized Couette viscometer. The apparatus for measuring both friction and viscosity is suitable for the minute volumes of synovial fluid available from clinical subjects. The findings will be relevant to the pathogenesis of cartilage abrasion in primary and secondary types of osteoarthritis. Conceivably, too, they may prove to be of diagnostic value.